As a conventional suspension apparatus for a vehicle, a torsion beam type rear suspension apparatus which has a torsion beam coupled between right and left rear wheels, and a pair of trailing arms which are formed into a planar shape to allow a displacement in the widthwise direction of the vehicle body and extend from the two end portions of the torsion beam in the longitudinal direction of the entire vehicle body is known (see Japanese Patent Laid-Open No. 4-283114). In this suspension apparatus, the free end portions of the trailing arms are supported on a vehicle body frame via bushings, and the rear end portions of the trailing arms are joined to the right and left end portions of the torsion beam by welding.
The torsion beam type rear suspension apparatus is mounted in a light car or the like since it has a simple structure and is inexpensive. Also, such rear suspension apparatus can decrease the floor height and can assure a broad rear cargo space or the like, since the coil spring level can lower by disposing the center of the coil diameter of each coil spring not immediately above the trailing arm or torsion beam but to have an offset on the front or rear side of the vehicle body with respect to the torsion beam. Hence, demands in recent wagon and box type vehicles are increasing.
However, in the technique of the aforementioned reference, when a spring seat or its bracket is formed at a corner portion, the actual effective length of the trailing arm becomes short, and torsional elasticity increases, resulting in poor riding comfort. For this reason, each trailing arm may be prolonged in correspondence with a decrease in effective length, but such long trailing arm disturbs layout efficiency. On the other hand, the torsion beam may be prolonged. However, the torsional elasticity of the torsion beam then becomes too low, resulting in poor driving stability, or the mechanical strength of that portion of the torsion beam on which the torsional stress is concentrated must be increased and may lead to high cost resulting from an increase in weight.
In the suspension apparatus in which the center of the coil diameter of each coil spring is disposed to have an offset on the front side of the vehicle body with respect to the torsion beam, since the torsion of the torsion beam is larger than that of the suspension apparatus in which the center of the coil diameter of each coil spring is disposed to have an offset on the rear side of the vehicle body, the separation of the joint surface between the spring seat and torsion beam becomes large. Even when their joint strength is increased, the torsional stress is concentrated, and the joint surface of the torsion beam cracks.